The present invention relates to the facilitation of orientation of thermotropic polymers in a magnetic field through the use of a dispersion of magnetic particles.
It is well established that both monomeric and polymeric liquid crystals are orientable in magnetic fields. See "Polymer Liquid Crystals", A. Cifferis et al., Academic press, 1982, pp. 275-308 and "The Physics of Liquid Crystals" Oxford Press 1975). High field strengths, on the order of 10.sup.3 -10.sup.4 Gauss, are normally required, however, restricting practical application.
The use of magnetic particles to facilitate orientation of monomeric liquid crystals of both the lyotropic (solution) and thermotropic variety has recently been reported. (See French Pat. App. 2462725. 1981, and Phys. Rev. Letters, 51, N. 25, 1983, p. 2298). In the latter work, a dispersion of elongated magnetic particles was utilized to substantially reduce the magnetic field strength required to orient the liquid crystal. This behavior has been predicted theoretically by Brochard and DeGennes Journal de Physique, 31, (1970) p. 691., and is attributed to a coupling of the magnetic particles with the liquid crystalline aggregates. The practical benefit is that the physical properties of the liquid crystalline system, such as its electrical or optical properties, may be varied with weak magnetic fields, thus enabling application, for example, in optical display devices. In the case of polymeric liquid crystals, it is anticipated that facilitation of orientation in weak magnetic fields will provide a route to reduced viscosity and/or controlled orientation in processing, for example, in injection-molding and in film or fiber extrusion. Control of orientation should result in improved control of the mechanical properties of the solid.